The present invention relates generally to electroluminescent (EL) display devices and particularly to EL displays comprised of a matrix of organic based light emitting diodes (LEDs) such as may be found in class 313/503, 504.
In recent years, several papers have appeared dealing with light emitting diodes in which the conventional semiconductor junction is replaced by one or two films made of organic materials. In some cases these materials are crystalline; in others they are polymers. A device of the latter type was described in the IEEE Transactions on Electron Devices, vol. 40, no. 7, July 1993. FIG. 1 illustrates the type of device discussed in the cited paper.
As seen in FIG. 1, the substrate 3 is a glass plate coated with a transparent layer of indium-tin oxide (ITO) 4 for the anode. The ITO anode has an electrical sheet resistance of ten ohms per square. First and second thin film organic layers of polymers 5, 6, designated TPD and Alq (see the original paper for chemical data), each forty nanometer thick, were successively deposited in vacuum. This was followed by a metal layer 7 serving as the cathode; in some experiments fifty nanometers of magnesium were used, in later experiments one nanometer of lithium. Finally, two-hundred nanometers of silver 8 were deposited on top of the magnesium or lithium layer to protect it from the atmosphere and serve as a cathode contact. After all the vacuum deposition procedures were completed the device was operated at room temperature and in the open atmosphere.
A maximum brightness of 15100 candelas (cd) per square meter was produced at a current density of 330 mA per square centimeter, with eighteen volts applied to the cell. When the Alq layer 6 was doped with coumarin, an organic dye, the brightness increased drastically to 40400 cd/square meter, without any significant change in electrical characteristics. The light output from the device in this last-mentioned condition was about twelve cd/ampere.
As might be expected, there has been considerable speculation about possible application of organic film diodes to flat panel displays for computers and for television. For example, an article on page 126 of the May 1993, Scientific American discusses that possibility in some detail. At first glance, it would appear that the new diodes would make the construction of flat panel matrix displays easy. The films are well suited to being deposited on flat substrates; large size panels seem possible because no single crystal wafers are involved; no vacuum spacers are needed as in field emission displays, and no critical distances need be maintained as in liquid crystal panels. Because diodes are inherently non-linear, there is no need for an .active matrix. Finally, operating voltages are low.
A flat panel, according to a simple extrapolation of the existing art, would merely require extending the light emitting films shown in FIG. 1 over a much larger surface. The light emitting films might well be made continuous across the entire area, while the conducting layers, i.e. ITO for the anode and metal layers for the cathode, would be subdivided into vertical and horizontal strips, respectively, to form the desired matrix.
However, there is a problem inherent in the design of a large matrix made up of light-emitting film diodes. It is this problem which the present invention addresses.
The figures quoted above, 40400 cd/square meter at 330 mA/square cm, represent a light/current ratio of about 12 candelas per ampere, a figure high enough to be of practical interest. The light is in the green portion of the spectrum which is the most visible. Let us examine the case of a panel with a 30 inch (76 cm) diagonal, having the conventional aspect ratio of 4:3.
The best known procedure for addressing a matrix of light emitting diodes is called "one-line-at-a-time". This means that if the matrix, for example, contains 480 horizontal rows or lines, only one of these rows is made to produce light at any given instant. If there are, for example, 640 pixels in each row, separate signals are applied simultaneously to each of 640 vertical conductors in order to produce the desired light intensity distribution in the particular row which is active at that instant. After a predetermined time interval, the selected row is turned off and the next row is turned on; each row has a duty factor (time on, divided by the total time) of one part in 480. It can be seen that each row is emitting light during only one part in 480 of the total time; so, if the luminance during that brief interval is 40400 cd per square meter, the time-averaged luminance as seen by a human observer is 40400 divided by 480, or 84 cd/square meter, equivalent to 25 footlamberts.
The area occupied by one horizontal row equals the total panel area (0.28 square meter) divided by the number of rows (480), or 5.83 square cm. Assuming that all pixels in the row are turned on, that row consumes an instantaneous current of 5.83 times 330 mA, or approximately two amperes. With the panel fully lighted, these same two amperes are switched in turn from row to row.
When one particular row is fully lighted, each pixel draws 1/640 of the total current, or about 3 mA, through its corresponding vertical conductor. Let us assume that the drive signals are applied to these conductors near the bottom of the panel. If the vertical conductors consisted only of ITO with 10 ohms per square, the series resistance of each conductor would vary from just a few ohms for a pixel in the bottom row to more than 5000 ohms for a pixel in the top row. At 3 mA, the latter figure would require an extra drive voltage of 15 volts, which is unacceptable; in practice, the voltage drop in the vertical conductor should not exceed 0.3 volt, which calls for vertical conductors having a resistance of no more than 100 ohms.
Let us now turn to the horizontal conductors. Assuming that the horizontal rows receive their drive power from the right edge of the panel, each horizontal conductor must be capable of carrying the full current of about 2 amperes and passing it from pixel to pixel without a significant voltage drop. As each pixel takes its share, about 3 mA, the current in the horizontal conductor decreases linearly from right to left, reaching zero at the left end. The brightness of each pixel is a strong function of the voltage applied to it; to prevent detrimental brightness changes from right to left, the voltage drop along the horizontal conductor should be kept to a small fraction of one volt. This is particularly important because any significant voltage drop caused by turned-on pixels on the right side of a row will reduce the brightness of all pixels on the left side of the same row. Thus, bright objects extended along the right half of the screen will produce conspicuous shadows to the left.
In a fully lighted row, as previously mentioned, the current in the horizontal conductor decreases linearly from its full value at one edge to zero on the other. The total voltage drop is therefore one-half the maximum current times the resistance of the conductor. A resistance of, for example, 0.2 ohms with a peak current of 2 ampere would give a maximum voltage drop of 0.2 volt which is acceptable. To achieve this low voltage drop a ribbon of copper, for example, 0.5 mm wide, 0.125 mm thick having a length equal to the width of the panel (61 cm) and having a resistance of about 0.17 ohms at room temperature would be suitable. It is evident, however, that a conductive ribbon of such thickness (125 um) cannot economically be formed by metal evaporation, the preferred method for preparing the thin film conducting layers shown in FIG. 1 or overlaid indelicately on the fragile LED thin film substrate.
The above discussion makes it clear that in a matrix type television display made up of light emitting diodes, the conductance requirements for vertical and horizontal conductors differ by at least three orders of magnitude, and, therefore, different manufacturing techniques are required for each type.